Abstract

This paper reviews recent progress on microwaveclocks using laser cooled neutral atoms. With an ultra-stable cryogenicsapphireoscillator as interrogation oscillator, a cesium fountain operates at the quantum projection noise limit. With
detected atoms, the relative frequency stability δν/ν is
where τ is the integration time in seconds. This stability is comparable to that of hydrogen masers. At
the measured stability reaches
Equally important is the accuracy of the frequency standard since
is the primary reference for the definition of the time unit, the second. The accuracy of our cesium fountain FO1 is presently
currently the best reported value.

A
fountain has also been constructed and the
ground-statehyperfine energy has been compared to the Cs primary standard with a relative accuracy of
Comparing the hyperfine energies of atoms with different atomic numbers Z, one can search for possible variations of the fine structure constant
with time. Measurements of the ratio
spread over an interval of 24 months indicate no change at a level of
placing a new upper limit for
The second attractive feature of
fountains is the smallness of the frequency shift induced by the mean field interaction between atoms. This shift is found to be at least ∼50 times below that of cesium.

Finally, the interest of the microgravity of space for cold atom experiments is out-lined. A space mission, ACES, carrying ultra-stable clocks, is presented. ACES has been selected by the European Space Agency to fly on the International Space Station in 2004.